Display device

ABSTRACT

One object is to provide a transistor including an oxide semiconductor film which is used for the pixel portion of a display device and has high reliability. A display device has a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode and a drain electrode over the oxide semiconductor film; a second gate insulating film over the source electrode, the drain electrode and the oxide semiconductor film; a second gate electrode over the second gate insulating film; an organic resin film having flatness over the second gate insulating film; a pixel electrode over the organic resin film having flatness, wherein the concentration of hydrogen atoms contained in the oxide semiconductor film and measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 .

TECHNICAL FIELD

The present invention relates to a display device having a transistor,and the channel of the transistor includes an oxide semiconductor film.

BACKGROUND ART

Patent Document 1 discloses a thin film transistor in which a first gateelectrode is formed on a substrate, a first gate insulating layer isformed so as to cover the first gate electrode, a semiconductor layerformed using an oxide semiconductor is formed over the first gateinsulating layer, a second gate insulating layer is formed over thesemiconductor layer, a second gate electrode is formed over the secondgate insulating layer, and a drain electrode and a source electrodewhich are connected to the semiconductor layer are formed, wherein thethickness of the second gate electrode is equal to or larger than thatof the first gate electrode is described (Claim 1). In Patent Document1, the above-described thin film transistor can be used for a drivingfield-effect transistor of a liquid crystal display or an organic ELdisplay (144th paragraph).

[REFERENCE]

-   [Patent Document 1] Japanese Published Patent Application No.    2009-176865

DISCLOSURE OF INVENTION

In Patent Document 1, the generation of a hump of conduction property ofa thin film transistor can be suppressed (26th paragraph).

However, the above-described structure is insufficient in order toimprove characteristics of a transistor. In view of the above, it is anobject of an embodiment of the present invention to provide a thin filmtransistor including an oxide semiconductor film, which is used for thepixel portion of a display device such as a liquid crystal display or anorganic EL display and has high reliability.

In the following experiment, the influence of hydrogen on a transistorincluding an oxide semiconductor film was examined. Thus, the inventorfound that characteristics of the transistor can be improved by removinghydrogen and the above problem can be solved.

<Experiment. Hydrogen and Transistor Characteristics>

FIGS. 1A and 1B show schematic diagrams of experimental transistorsincluding oxide semiconductor films (amorphous In—Ga—Zn—O films, in thisexperiment, also referred to as a-IGZO) and characteristics of thetransistors. FIG. 1A shows the case where SiO_(x) formed by a plasma CVDmethod (also referred to as PECVD-SiO_(x)) is used for an interlayerfilm and FIG. 1B shows the case where SiO_(x) formed by a sputteringmethod (also referred to as sputtered-SiO_(x) or sputtering-SiO_(x)) isused for the interlayer film. The manufacturing method other than theabove is the same in each of the transistors. When SiO_(x), formed by aplasma CVD method was used, normally-on transistor characteristics wereobtained. In addition, the change in transistor characteristicsdepending on the measurement temperature was large. On the other hand,when SiO_(x), formed by a sputtering method was used, normally-offtransistor characteristics were obtained and the change in transistorcharacteristics depending on the measurement temperature was small. Whenthe hydrogen concentration of each of these two transistors was measuredby secondary ion mass spectrometry, it was found that the transistor inwhich SiO_(x) formed by a plasma CVD method is used includes a largeamount of hydrogen; however, the transistor in which SiO_(x) formed by asputtering method is used includes a small amount of hydrogen (FIG. 2).

Further, in order to clarify electronic characteristics of a-IGZO,analysis thereof was conducted by first principle calculation. (A)a-IGZO satisfying the stoichiometric proportion was and (B) a-IGZO towhich hydrogen was added were assumed, and the electronic states werecalculated. A unit cell of 84 atoms, the composition ratio ofIn:Ga:Zn:O=1:1:1:4, and the density of 5.9 g/cm³ was assumed, and anamorphous structure was reproduced by the classical molecular dynamicsmethod, and further, optimization of the structure was performed by thequantum molecular dynamic method. Then, the electronic states werecalculated.

The calculation results are shown in FIGS. 3A and 3B. FIGS. 3A and 3Bshow DOS (Density Of States, the electron density of states) of a-IGZO.The point where Energy shows 0 (zero) is the Fermi level. It is foundthat, in (A) a-IGZO satisfying the stoichiometric proportion, the Fermilevel exists in the valence band; however, in (B) a-IGZO to whichhydrogen is added, electrons exist also in the conduction band. That is,it is found that, in a-IGZO, hydrogen forms a donor level and supplieselectrons.

Removing hydrogen is removing a donor in the oxide semiconductor film.The oxide semiconductor film can be a semiconductor which is madeintrinsic or substantially intrinsic by removing the donor.

When the off-state current of the transistor (the channel length(L)=10.0 μm, the channel width (W)=1 m) whose channel has the oxidesemiconductor film with reduced hydrogen concentration was measured atroom temperature (25° C.), the off-state current was equal to or lowerthan 1×10⁻¹² A (FIG. 4). When W is converted into 1 μm, the off-statecurrent is equal to or lower than 1×10⁻¹⁸ A.

The oxide semiconductor film can be a semiconductor which is madeintrinsic or substantially intrinsic by minimizing the number ofhydrogen atoms contained in the oxide semiconductor film. Accordingly,characteristics of the transistor can be improved and the above problemcan be solved.

An embodiment of the present invention is a display device having afirst gate electrode; a first gate insulating film over the first gateelectrode; an oxide semiconductor film over the first gate insulatingfilm; a source electrode and a drain electrode over the oxidesemiconductor film, and wherein the source electrode and the drainelectrode are electrically connected to the oxide semiconductor film; asecond gate insulating film over the source electrode, the drainelectrode, and the oxide semiconductor film; a second gate electrodeover the second gate insulating film, and wherein the second gateelectrode is electrically connected to the first gate electrode; anorganic resin film having flatness over the second gate insulating film;a pixel electrode over the organic resin film having flatness, andwherein the pixel electrode is electrically connected to either thesource electrode or the drain electrode; a first alignment film over andin contact with the second gate electrode and the pixel electrode; aliquid crystal layer over the first alignment film; a second alignmentfilm over the liquid crystal layer; a counter electrode over the secondalignment film; and a counter substrate over the counter electrode,wherein the concentration of hydrogen atoms contained in the oxidesemiconductor film and measured by secondary ion mass spectrometry isless than 1×10¹⁶ cm³.

An embodiment of the present invention is a display device having afirst gate electrode; a first gate insulating film over the first gateelectrode; an oxide semiconductor film over the first gate insulatingfilm; a source electrode and a drain electrode over the oxidesemiconductor film, and wherein the source electrode and the drainelectrode are electrically connected to the oxide semiconductor film; asecond gate insulating film over the source electrode, the drainelectrode and the oxide semiconductor film; a second gate electrode overthe second gate insulating film, and wherein the second gate electrodeis electrically connected to the first gate electrode; an organic resinfilm having flatness over the second gate insulating film; a pixelelectrode over the organic resin film having flatness, and wherein thepixel electrode is electrically connected to either the source electrodeor the drain electrode; an EL layer over the pixel electrode; a counterelectrode over the EL layer; a sealing material over and in contact withthe second gate electrode and the counter electrode; and a countersubstrate over the sealing material, wherein the concentration ofhydrogen atoms contained in the oxide semiconductor film and measured bysecondary ion mass spectrometry is less than 1×10¹⁶ cm³.

An embodiment of the present invention is a display device having afirst gate electrode; a first gate insulating film over the first gateelectrode; an oxide semiconductor film over the first gate insulatingfilm; a source electrode and a drain electrode over the oxidesemiconductor film, and wherein the source electrode and the drainelectrode are electrically connected to the oxide semiconductor film; asecond gate insulating film over the source electrode, the drainelectrode and the oxide semiconductor film; a second gate electrode overthe second gate insulating film, and wherein the second gate electrodeis electrically connected to the first gate electrode; an organic resinfilm having flatness over the second gate insulating film; a pixelelectrode over the organic resin film having flatness, and wherein thepixel electrode is electrically connected to either the source electrodeor the drain electrode; and a filler over and in contact with the secondgate electrode and the pixel electrode, wherein a spherical particleincluding a cavity is provided in the filler, the cavity contains ablack region and a white region, and a space around the cavity is filledwith a liquid, wherein the concentration of hydrogen atoms contained inthe oxide semiconductor film and measured by secondary ion massspectrometry is less than 1×10¹⁶ cm⁻³.

An embodiment of the present invention is a display device having afirst gate electrode; a first gate insulating film over the first gateelectrode; an oxide semiconductor film over the first gate insulatingfilm; a source electrode and a drain electrode over the oxidesemiconductor film, and wherein the source electrode and the drainelectrode are electrically connected to the oxide semiconductor film; asecond gate insulating film over the source electrode, the drainelectrode and the oxide semiconductor film; a second gate electrode overthe second gate insulating film, and wherein the second gate electrodeis electrically connected to the first gate electrode; an organic resinfilm having flatness over the second gate insulating film; a pixelelectrode over the organic resin film having flatness, and wherein thepixel electrode is electrically connected to either the source electrodeor the drain electrode; and an electronic ink layer over and in contactwith the second gate electrode and the pixel electrode, wherein amicrocapsule in which positively charged white microparticles andnegatively charged black microparticles are encapsulated is provided inthe electronic ink layer, wherein the concentration of hydrogen atomscontained in the oxide semiconductor film and measured by secondary ionmass spectrometry is less than 1×10¹⁶ cm⁻³.

According to an embodiment of the present invention, the concentrationof hydrogen atoms measured by secondary ion mass spectrometry of theoxide semiconductor film in which a channel formed is less than 1×10¹⁶cm³. Accordingly, the oxide semiconductor film is a semiconductor whichis made intrinsic or substantially intrinsic. The carrier density of theoxide semiconductor film is extremely reduced. The off-state current ofthe transistor is extremely decreased. Further, avalanche breakdown isnot easily generated.

Furthermore, as Patent document regarding the concentration of hydrogenatoms, Japanese Published Patent Application No. 2007-103918 is given.In Japanese Published Patent Application No. 2007-103918, a field-effecttransistor having a channel layer formed of an amorphous oxide filmincluding In or Zn, wherein the amorphous oxide film includes hydrogenatoms or deuterium atoms of equal to or greater than 10¹⁶ cm⁻⁶ and equalto or less than 10²⁰ cm⁻³, is described. However, hydrogen atoms arepositively added, which is a technical idea of the above document. Onthe other hand, the number of hydrogen atoms is minimized, which is atechnical idea of an embodiment of the present invention. Therefore,these technical ideas are opposite and completely different from eachother. According to an embodiment of the present invention, theconcentration of hydrogen atoms contained in the oxide semiconductorfilm is less than 1×10¹⁶ cm⁻³, whereby the oxide semiconductor film canbe a semiconductor which is made intrinsic or substantially intrinsic.Moreover, the density of the carriers can be extremely reduced, and theoff-state current of the transistor can be extremely decreased.

According to an embodiment of the present invention, the transistorwhich is electrically connected to the pixel electrode is not coveredwith the organic resin film having flatness and the pixel electrode.Hydrogen atoms of the organic resin film do not adversely affect thetransistor. The potential applied to the pixel electrode does notadversely affect the transistor.

According to an embodiment of the present invention, the second gateinsulating film is formed over the source electrode and the drainelectrode. Therefore, the source electrode and the drain electrode areprotected by the second gate insulating film.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic diagrams of transistors and diagramsshowing characteristics thereof.

FIG. 2 shows a hydrogen concentration profile of a transistor.

FIGS. 3A and 3B show calculation results.

FIG. 4 shows transistor characteristics.

FIGS. 5A and 5B illustrate a display device.

FIGS. 6A and 6B illustrate a display device.

FIG. 7 is a cross sectional view of a transistor.

FIGS. 8A and 8B are energy band diagrams of a transistor.

FIGS. 9A and 9B are energy band diagrams of a transistor.

FIG. 10 is an energy band diagram of a transistor.

FIGS. 11A to 11C illustrate a manufacturing method of a display device.

FIGS. 12A and 12B illustrate a manufacturing method of a display device.

FIG. 13 illustrates a method for manufacturing a display device.

FIGS. 14A and 14B each illustrate a display device.

FIGS. 15A and 15B each illustrate a display device.

FIGS. 16A and 16B each illustrate a display device.

FIG. 17 shows a C-V measurement.

FIGS. 18A and 18B show a C-V measurement.

FIGS. 19A and 19B each show a TEM image at a cross section.

FIGS. 20A and 20B each show a TEM image at a cross section.

FIG. 21A shows an enlarged view of the superficial portion of the sampleA and FIGS. 21B to 21F each illustrate an electron diffraction patternof a crystalline region.

FIGS. 22A and 22B each illustrate a display device.

FIGS. 23A and 233 show a mobile phone and a portable informationterminal, respectively.

FIGS. 24A and 24B illustrate a television set and a digital photo frame,respectively.

FIG. 25 illustrates a portable amusement machine.

FIG. 26 illustrates an electronic book.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below. However,the present invention can be carried out in many different modes, and itis easily understood by those skilled in the art that modes and detailsof the present invention can be modified in various ways withoutdeparting from the purpose and the scope of the present invention.Therefore, the present invention is not construed as being limited todescription of the embodiment. Note that the same reference numerals arecommonly given to the same portions or portions having similar functionsin different drawings, and repetitive explanation will be omitted insome cases.

[Embodiment 1]

This embodiment discloses a display device including a first gateelectrode; a first gate insulating film formed over the first gateelectrode; an oxide semiconductor film formed over the first gateinsulating film; and a source electrode and a drain electrode formedover the oxide semiconductor film. The source electrode and the drainelectrode are electrically connected to the oxide semiconductor film. Asecond gate insulating film formed over the source electrode, the drainelectrode, and the oxide semiconductor film; and a second gate electrodeformed over the second gate insulating film are provided. The secondgate electrode is electrically connected to the first gate electrode. Anorganic resin film having flatness formed over the second gateinsulating film and a pixel electrode formed over the organic resin filmhaving flatness are provided. The pixel electrode is electricallyconnected to either the source electrode or the drain electrode, and adisplay medium formed over and in contact with the second gate electrodeand the pixel electrode.

As illustrated in FIGS. 5A and 5B, the display device includes a pixel20. The display device may include a plurality of pixels 20. The pixel20 includes a transistor 15. The transistor 15 includes a first gateelectrode 3, a first gate insulating film 4, an oxide semiconductor film5, an electrode 6A (one of a source electrode and a drain electrode), anelectrode 6B (the other of the source electrode and the drainelectrode), a second gate insulating film 7, and a second gate electrode8. The pixel 20 includes an organic resin film 9 having flatness and apixel electrode 10. FIG. 5A is a top view of the pixel 20 and FIG. 5Billustrates a cross section A-B and a cross section C-D in FIG. 5A.

The transistor 15 is formed over a substrate 1. An insulating film 2serving as a base film is formed over the substrate 1. The first gateelectrode 3 is formed over the insulating film 2. The first gateinsulating film 4 is formed over the first gate electrode 3 to cover thefirst gate electrode 3. The oxide semiconductor film 5 is formed overthe first gate insulating film 4. The electrode 6A and the electrode 6Bare formed over the oxide semiconductor film 5. The second gateinsulating film 7 is formed over the electrode 6A, the electrode 6B, andthe oxide semiconductor film 5. The second gate insulating film 7 coversthe electrode 6A, the electrode 6B, and the oxide semiconductor film 5.The second gate electrode 8 is formed over the second gate insulatingfilm 7.

In FIG. 5A, the second gate electrode 8 is electrically connected to thefirst gate electrode 3; however, the second gate electrode 8 is notnecessarily to be electrically connected to the first gate electrode 3(FIGS. 6A and 6B). When the second gate electrode 8 is electricallyconnected to the first gate electrode 3, the second gate electrode 8 hasthe same potential as the first gate electrode 3. On the other hand,when the second gate electrode 8 is not electrically connected to thefirst gate electrode 3, the second gate electrode 8 does not always havethe same potential as the first gate electrode 3.

The first gate electrode 3 is electrically connected to a scan linedriver circuit (not shown). A selection signal from the scan line drivercircuit is applied to the first gate electrode 3. When the first gateelectrode 3 is electrically connected to the second gate electrode 8,the same selection signal is also applied to the second gate electrode8. When the first gate electrode 3 is not electrically connected to thesecond gate electrode 8, the second gate electrode 8 is electricallyconnected to another scan line driver circuit and another selectionsignal from the scan line driver circuit is applied thereto.

The electrode 6A is electrically connected to a signal line drivercircuit (not shown). An image signal from the signal line driver circuitis applied to the electrode 6A.

The organic resin film 9 having flatness is formed over the second gateinsulating film 7. The organic resin film 9 is not formed over thesecond gate electrode 8. The pixel electrode 10 is formed over theorganic resin film 9. A display medium is formed over the second gateelectrode 8, the second gate insulating film 7, and the pixel electrode10 (FIGS. 14A, 14B, 15A, 15B, 16A, 16B, 22A and 22B).

A storage capacitor is not provided for a display device in thisembodiment. Since the off-state current of the transistor 15 isextremely low, the potentials which are applied to the electrode 6B andthe pixel electrode 10 are held without decreasing. Therefore, a storagecapacitor is not needed. However, it is needless to say that a storagecapacitor may be provided.

The first gate insulating film 4 and the second gate insulating film 7may be in contact with each other in a region where neither theelectrode 6A nor the electrode 6B is formed (FIGS. 5B, 6B, and 7). Whenthe first gate insulating film 4 and the second gate insulating film 7are in contact with each other, the oxide semiconductor film 5 iscovered with the first gate insulating film 4 and the second gateinsulating film 7, whereby impurities can be prevented from entering theoxide semiconductor film 5.

Features of a display device are described below. The concentration ofhydrogen atoms contained in the oxide semiconductor film 5 in which achannel is formed is less than 1×10¹⁶ cm⁻³. Therefore, the oxidesemiconductor film 5 is a semiconductor which is intrinsic orsubstantially intrinsic. Descriptions thereof will be made later.

The electrode 6A and the electrode 6B are covered with the second gateinsulating film 7. The electrode 6A and the electrode 6B are protectedby the second gate insulating film 7.

As illustrated in FIG. 5B and FIG. 6B, the organic resin film 9 and thepixel electrode 10 are not formed over the transistor 15. Hydrogen atomscontained in the organic resin film 9 do not affect the transistor 15.The potential which is applied to the pixel electrode 10 does not affectthe transistor 15.

Next, each component of the display device is described below.

First, the oxide semiconductor film 5 is described. A channel is formedin the oxide semiconductor film 5. The oxide semiconductor film 5 may beformed to have a thickness of 2 nm to 200 nm.

(Composition)

As a material of the oxide semiconductor film 5, any of the followingoxide semiconductors can be used: a four-component metal oxide such asIn—Sn—Ga—Zn—O; a three-component metal oxide such as In—Ga—Zn—O,In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O, or Sn—Al—Zn—O; atwo-component metal oxide such as In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O,Sn—Mg—O, or In—Mg—O; In—O, Sn—O, Zn—O, or the like. Further, SiO₂ may becontained in the above oxide semiconductor.

As the oxide semiconductor film 5, a thin film expressed by InMO₃(ZnO)_(m)(m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like. Among the oxide semiconductorfilm 5 expressed by InMO₃ (ZnO)_(m) (m>0), an oxide semiconductor whichincludes Ga as M is referred to as an In—Ga—Zn—O oxide semiconductor,and a thin film of the In—Ga—Zn—O oxide semiconductor is also referredto as an In—Ga—Zn—O film.

(Hydrogen Concentration)

The hydrogen concentration of the oxide semiconductor film 5 measured bysecondary ion mass spectrometry is less than 1×10¹⁶ cm⁻³, preferablyless than 1×10¹⁴ cm⁻³. As described above, the oxide semiconductor 5with reduced hydrogen concentration is a semiconductor which is madeintrinsic or substantially intrinsic. Further, it is preferable that theoxide semiconductor 5 does not include an impurity element forming adonor level or an acceptor level.

It is known that it is difficult to obtain data in the proximity of asurface of a sample or in the proximity of an interface between stackedfilms formed using different materials by secondary ion massspectrometry analysis in principle. Thus, in the case where distributionof the hydrogen concentration of a film in thickness direction isanalyzed by secondary ion mass spectrometry, an average value in aregion where the film is provided, the value is not greatly changed, andalmost the same intensity can be obtained is employed as the hydrogenconcentration. Further, in the case where the thickness of the film issmall, a region where almost the same intensity can be obtained cannotbe found in some cases due to the influence of the hydrogenconcentration of the film adjacent thereto. In this case, the maximumvalue or the minimum value of the hydrogen concentration of a regionwhere the film is provided is employed as the hydrogen concentration ofthe film. Furthermore, in the case where a mountain-shaped peak havingthe maximum value and/or a valley-shaped peak having the minimum valuedo/does not exist in the region where the film is provided, the value ofthe inflection point is employed as the hydrogen concentration.

The number of hydrogen atoms contained in the oxide semiconductor film 5is preferably zero. The concentration of hydrogen atoms is equal to orgreater than 0 cm⁻³ and less than 1×10¹⁶ cm⁻². However, it is difficultto detect that the number of hydrogen atoms is zero by secondary ionmass spectrometry. Therefore, it is preferable that the concentration ofhydrogen atoms is at least equal to or less than the detection limit ofsecondary ion mass spectrometry.

(Carrier Density)

The carrier density of the oxide semiconductor film 5 having theconcentration of hydrogen atoms as described above is less than 1×10¹²cm⁻³ at 300 K. The carrier density of the oxide semiconductor film 5 maybe less than 1×10¹² cm⁻² at 300 K. It is preferable that the carrierdensity of the oxide semiconductor film 5 is less than 1×10¹⁰ cm⁻³ at300 K. The carrier density can be estimated by the Hall measurement, orthe like. The measuring method used here is described in Examples.

Here, the intrinsic carrier density of an oxide semiconductor isdescribed. The intrinsic carrier density refers to the carrier densityof an intrinsic semiconductor.

The intrinsic carrier density contained in a semiconductor, n_(i), canbe obtained by approximating Fermi-Dirac distribution according toFermi-Dirac statistics with the formula of Boltzmann distribution (seeFormula 1).

$\begin{matrix}{n_{i} = {\sqrt{N_{C}N_{V}}{\exp\left( {- \frac{E_{g}}{2{kT}}} \right)}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The intrinsic carrier density, n_(i), can be obtained by theapproximation formula is a relational expression of density of aneffective state at a conduction band, N_(c), density of an effectivestate at a valence band, N_(v), and a band gap, E_(g). According to theabove Formula, the intrinsic carrier density, of silicon is 1.4×10¹⁰cm⁻³ and the intrinsic carrier density, n_(i), of an oxide semiconductor(here, In—Ga—Zn—O film) is 1.2×10⁻⁷ cm⁻³. It is found that the intrinsiccarrier density of the oxide semiconductor is extremely lower than thatof silicon.

The carrier density of the oxide semiconductor film 5 having theconcentration of hydrogen atoms as described above is extremely low andthe off-state current of the transistor 15 including the oxidesemiconductor film 5 is extremely low.

Next, structures other than the oxide semiconductor film 5 aredescribed.

The substrate 1 has enough heat resistance to withstand heat treatmentto be performed later. As the substrate 1, a glass substrate of bariumborosilicate glass, aluminoborosilicate glass, or the like is used.Alternatively, a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, a sapphire substrate, or a crystallizedglass substrate may be used as the substrate 1. Further alternatively, aplastic film or the like formed using polyethylene terephthalate,polyimide, an acrylic resin, polycarbonate, polypropylene, polyester,polyvinyl chloride, or the like can be used as long as it has heatresistance high enough to withstand heat treatment to be performedlater.

For the insulating film 2, a silicon oxide film, a silicon nitride film,a silicon oxynitride film, or the like is used. It is preferable thatsubstances including hydrogen atoms such as hydrogen, hydroxyl, ormoisture are not contained in the insulating film 2. The insulating film2 may be formed to have a thickness of 10 nm to 200 nm. The insulatingfilm 2 prevents impurities contained in the substrate 1 from enteringthe first gate insulating film 4 and the oxide semiconductor film 5.Note that the insulating film 2 is not necessarily to be formed if it isnot necessary to consider impurities contained in the substrate 1.

The first gate electrode 3 is formed in a single layer or a stackedlayer using a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, or scandium, or analloy material which includes any of these materials as a maincomponent. It is preferable that substances including hydrogen atomssuch as hydrogen, hydroxyl, or moisture are not contained in the firstgate electrode 3. The first gate electrode 3 may be formed to have athickness of 10 nm to 200 nm.

The first gate insulating film 4 is formed in a single layer or astacked layer using a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, a hafnium silicate (HfSiO_(x)) film, a HfSi₃O_(y) film to which Nis added, a hafnium aluminate (HfAlO_(x)) film to which nitrogen isadded, a hafnium oxide film, and/or an yttrium oxide film. By using ahigh-k material such as a hafnium silicate (HfSiO_(x)) film, aHfSi₃O_(y) film to which N is added, a hafnium aluminate (HfAlO_(x))film to which nitrogen is added, a hafnium oxide film, or an yttriumoxide film, gate leakage can be reduced. It is preferable thatsubstances including hydrogen atoms such as hydrogen, hydroxyl, ormoisture are not contained in the first gate insulating film 4. Thefirst gate insulating film 4 may be formed to have a thickness of 10 nmto 500 nm.

A halogen element such as fluorine or chlorine may be contained in thefirst gate insulating film 4 at a concentration of about 5×10¹⁸ cm⁻³ to1×10²⁰ cm⁻³. Substances including hydrogen atoms such as hydrogen,moisture, hydroxyl, or hydride which may be contained in the oxidesemiconductor film 5 or the interface between the first gate insulatingfilm 4 and the oxide semiconductor film 5 can be removed by such ahalogen element. When as the first gate insulating film 4, for example,a stacked layer of a silicon nitride film and a silicon oxide film isformed, it is preferable that the silicon oxide film contain a halogenelement at the above concentration and is disposed on the side incontact with the oxide semiconductor film 5. The silicon nitride filmprevents impurities such as hydrogen, moisture, hydroxyl, or hydride(also referred to as a hydrogen compound) from entering the siliconoxide film.

The electrode 6A (one of a source electrode and a drain electrode) andthe electrode 6B (the other of the source electrode and the drainelectrode) are formed in a single layer or a stacked layer using a metalmaterial such as molybdenum, titanium, chromium, tantalum, tungsten,aluminum, copper, neodymium, scandium, or yttrium; an alloy materialwhich includes any of these materials as a main component; alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is preferable that hydrogen, hydroxyl,or moisture is not contained in the electrode 6A and the electrode 6B.The electrode 6A and the electrode 6B may be formed to have thicknessesof 10 nm to 500 nm.

The second gate insulating film 7 can be formed using the same materialas the first gate insulating film 4. For example, a silicon oxide filmis used. It is preferable that hydrogen, hydroxyl, or moisture is notcontained in the second gate insulating film 7. The second gateinsulating film 7 may be formed to have a thickness of 10 nm to 200 nm.Note that a silicon oxynitride film, an aluminum oxide film, an aluminumoxynitride film, or the like can be used instead of the silicon oxidefilm.

Further, the second gate insulating film 7 may include a defect. Thedefect can capture hydrogen atoms contained in the oxide semiconductorfilm 5. Thus, the number of hydrogen atoms contained in the oxidesemiconductor film 5 can be further reduced.

Halogen elements such as fluorine or chlorine may be contained in thesecond gate insulating film 7 at a concentration of about 5×10¹⁸ cm⁻³ to1×10²⁰ cm⁻³. Impurities such as hydrogen, moisture, hydroxyl, or hydridewhich may be contained in the oxide semiconductor film 5 or at theinterface between the second gate insulating film 7 and the oxidesemiconductor film 5 can be removed by such a halogen element.

The second gate electrode 8 is formed as a single layer or a stackedlayer using a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloymaterial which contains any of these materials as a main component. Itis preferable that substances including hydrogen atoms such as hydrogen,hydroxyl, or moisture are not contained in the second gate electrode 8.The second gate electrode 8 may be formed to have a thickness of 10 nmto 200 nm. The second gate electrode 8 may be formed to be thicker thanthe first gate electrode 3. The second gate electrode 8 may be formedusing the same material as the first gate electrode 3.

The organic resin film 9 is formed using an acrylic resin film havingflatness, a polyimide resin film having flatness, or the like. Theorganic resin film 9 may be formed to have a thickness of 1.0 μm to 2.0μm.

For the pixel electrode 10, typically, a conductive material havingreflectance or light-blocking properties, such as a single materialformed using an element selected from aluminum, copper, titanium,tantalum, tungsten, molybdenum, chromium, neodymium, or scandium; analloy containing any of these elements; a compound (e.g., an oxide or anitride) containing any of these elements; or the like can be used.Alternatively, as the conductive material having a light-transmittingproperty, a conductive material having a light-transmitting property,such as indium tin oxide, indium oxide including tungsten oxide, indiumzinc oxide including tungsten oxide, indium oxide including titaniumoxide, indium tin oxide including titanium oxide, indium zinc oxide, orindium tin oxide to which silicon oxide is added, can be used. A stackedstructure containing any of these materials can also be used. The pixelelectrode 10 may be formed to have a thickness of 50 nm to 500 nm.

Next, the operation of the transistor 15 having the oxide semiconductorfilm 5 will be described with reference to energy band diagrams.

FIG. 7 is a cross-sectional view of a common inverted staggered thinfilm transistor using an oxide semiconductor film. An oxidesemiconductor film (OS) is formed over a gate electrode (GE) with afirst gate insulating film (GI) interposed therebetween. A sourceelectrode (S) and a drain electrode (D) are formed over the oxidesemiconductor film and the first gate insulating film. A passivationfilm (PI) is formed over the source electrode and the drain electrode.In a region where neither the source electrode nor the drain electrodeis not formed, the first gate insulating film and the passivation filmare in contact with each other.

FIGS. 8A and 8B are energy band diagrams (schematic diagrams) of across-section taken along line A-A′ in FIG. 7. FIG. 8A illustrates thecase where the potential of a voltage applied to the source is equal tothe potential of a voltage applied to the drain (VD=0 V), and FIG. 8Billustrates the case where a positive potential is applied to the drain(VD>0) with respect to the source.

FIGS. 9A and 9B are energy band diagrams (schematic diagrams) of across-section taken along line B-B′ in FIG. 7. FIG. 9A illustrates astate where a positive potential (+V_(G)) is applied to a gate electrode(GE) and an on state where carriers (electrons) flow between the sourceelectrode and the drain electrode. FIG. 9B illustrates a state where anegative potential (−V_(G)) is applied to the gate electrode (GE) and anoff state (a minority carrier does not flow).

FIG. 10 illustrates a relation between a work function (φ_(M)) of avacuum level and a metal and electron affinity (χ) of an oxidesemiconductor.

Because metal is degenerated, the conduction band and the Fermi levelcorrespond to each other. On the other hand, a conventional oxidesemiconductor is typically an n-type semiconductor, in which case theFermi level (Ef) is away from the intrinsic Fermi level (Ei) located inthe middle of a band gap and is located closer to the conduction band.Note that it is known that hydrogen is a donor in an oxide semiconductorand is one factor causing an oxide semiconductor to be an n-typesemiconductor.

In contrast, the oxide semiconductor film in this embodiment is made anintrinsic (i-type) or substantially intrinsic oxide semiconductor filmobtained by removal of hydrogen atoms, which is n-type impurities, froman oxide semiconductor film to increase the purity so that an impurityother than a main component of the oxide semiconductor film includes aslittle as possible. In other words, a feature is that a highly-purifiedi-type (an intrinsic semiconductor), or a semiconductor close thereto,is obtained not by adding an impurity but by removing hydrogen atoms asmuch as possible. In this manner, the Fermi level (Ef) can be the samelevel as the intrinsic Fermi level (Ei).

In the case where the band gap (Eg) of an oxide semiconductor is 3.15eV, the electron affinity (χ) is said to be 4.3 eV. When the sourceelectrode and the drain electrode are formed using titanium, forexample, the work function of titanium is substantially equal to theelectron affinity (χ) of the oxide semiconductor. In that case, aSchottky barrier for electrons is not formed at an interface between themetal and the oxide semiconductor.

In other words, in the case where the work function of metal (φ_(M)) andthe electron affinity (χ) of the oxide semiconductor are equal to eachother and the metal and the oxide semiconductor are in contact with eachother, an energy band diagram (a schematic diagram) as illustrated inFIG. 8A is obtained.

In FIG. 8B, a black circle (•) represents an electron, and when apositive potential is applied to the drain, the electron is injectedinto the oxide semiconductor film over the barrier (h) and flows towardthe drain. In that case, the height of the barrier (h) is changeddepending on gate voltage and drain voltage. When positive drain voltageis applied, the height (h) of the barrier is smaller than the height ofthe barrier of FIG. 8A without application of voltage, that is, ½ of theband gap (Eg).

At this time, the electron moves in the bottom, which is energeticallystable, on the oxide semiconductor film side at the interface betweenthe gate insulating film and the highly purified oxide semiconductorfilm as illustrated in FIG. 9A.

In FIG. 9B, when a negative potential (reverse bias) is applied to thegate electrode (GE), holes which are minority carriers are substantiallyzero; therefore, current is substantially close to zero.

Further, the band gap (Eg) of an oxide semiconductor is approximatelythree times larger than that of silicon in the case where the band gap(Eg) of the oxide semiconductor is 3.15 eV. The oxide semiconductorhaving such an Eg is resistant to an avalanche breakdown. In atransistor using an oxide semiconductor film in which the number ofhydrogen atoms is reduced, voltage applied to a drain electrode can belarger than that of a transistor using an oxide semiconductor film inwhich the number of hydrogen atoms is not reduced or a transistor usingsilicon.

As described above, by the oxide semiconductor film 5 in which thenumber of hydrogen atoms is reduced, the operation of the transistor 15can be favorable.

Next, a method for manufacturing a display device is described.

The insulating film 2 serving as a base film is formed over thesubstrate 1 by a plasma CVD method, a sputtering method, or the like(FIG. 11A). Note that the insulating film 2 is preferably formed by asputtering method in order not to include a large amount of hydrogen.

An example in which a silicon oxide film is formed is described. Asputtering gas containing high purity oxygen, from which hydrogen,water, hydroxyl, or hydride (also referred to as a hydrogen compound),or the like is removed, is introduced, and a silicon semiconductortarget is used, whereby a silicon oxide film is formed over thesubstrate 1. The temperature of the substrate 1 may be room temperature,or the substrate 1 may be heated.

Alternatively, a silicon oxide film is formed as follows: quartz(preferably synthesized quart) is used as the target; the substratetemperature is 108° C.; the distance between the target and thesubstrate is 60 mm; the pressure is 0.4 Pa; the high-frequency power is1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen toargon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. As asputtering gas, oxygen or a mixed gas of oxygen and argon is used.

The insulating film 2 is preferably formed removing moisture remainingin the deposition chamber. This is for preventing hydrogen, water,hydroxyl, hydride, or the like from being contained in the insulatingfilm 2.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber in which exhaustion is performed with the use of acryopump, hydrogen, water, hydroxyl, hydride, or the like, is exhausted.Accordingly, the concentration of impurities included in the insulatingfilm 2 formed in the deposition chamber can be reduced.

It is preferable to use a high-purity gas from which an impurity such ashydrogen, water, hydroxyl, or hydride is removed to a level of parts permillion or parts per billion, as a sputtering gas.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power supply, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Furthermore, as a deposition method using a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringmethod in which voltage is also applied to a substrate duringdeposition.

As the sputtering in this specification, the above-described sputteringdevice and the sputtering method can be employed as appropriate.

In the case where a silicon nitride film and a silicon oxide film arestacked to form the insulating film 2, the silicon nitride film and thesilicon oxide film are formed in the same deposition chamber with theuse of a common silicon target. First, a sputtering gas includingnitrogen is introduced, and a silicon target provided in the depositionchamber is used, whereby the silicon nitride film is formed. Then, thesputtering gas is switched to a sputtering gas including oxygen, and thesame silicon target is used, whereby the silicon oxide film is formed.The silicon nitride film and the silicon oxide film can be formed insuccession without being exposed to air; therefore, impurities such ashydrogen, water, hydroxyl, or hydride can be prevented from beingadsorbed on the surface of the silicon nitride film.

A conductive film is formed over the insulating film 2, and then etchingis performed on the conductive film with the use of a resist mask formedin a photolithography process, so that the first gate electrode 3 isformed (FIG. 11A). Note that the conductive film is preferably formed bya sputtering method in order to prevent hydrogen, hydroxyl, or moisturefrom being contained in the conductive film. An end portion of the firstgate electrode 3 preferably has a tapered shape because coverage withthe first gate insulating film 4 stacked later can be improved.

The first gate insulating film 4 is formed over the first gate electrode3. Note that the first gate insulating film 4 is preferably formed by asputtering method in order to prevent hydrogen, hydroxyl, or moisturefrom being contained in the conductive film. Therefore, as pretreatmentfor deposition, it is preferable that the substrate 1 provided with thefirst gate electrode 3 be preheated in a preheating chamber of asputtering apparatus and impurities such as hydrogen, water, hydroxyl,or hydride adsorbed on the substrate 1 be eliminated and evacuated. Thepreheat temperature is equal to or greater than 100° C. and equal to orless than 400° C., preferably equal to or greater than 150° C. and equalto or less than 300° C. Note that as an evacuation means, a cryopump ispreferably provided in the preheating chamber. Note that this preheatingtreatment can be omitted.

For example, in the case where a silicon oxide film is formed as thefirst gate insulating film 4, a silicon target or a quartz target isused as a target, and oxygen or a mixed gas of oxygen and argon is usedas a sputtering gas.

The oxide semiconductor film is formed over the first gate insulatingfilm 4 by a sputtering method. Note that before the oxide semiconductorfilm is formed by a sputtering method, powdery substances (also referredto as particles or dust) attached on a surface of the first gateinsulating film 4 are preferably removed by reverse sputtering in whichan argon gas is introduced and plasma is generated. The reversesputtering refers to a method in which, without application of voltageto a target side, a high-frequency power source is used for applicationof voltage to a substrate side in an argon atmosphere to generate plasmain the vicinity of the substrate and modify a surface. Note that insteadof an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, anoxygen atmosphere, or the like may be used.

A target including an oxide semiconductor is used. For example, a targetof a metal oxide containing zinc oxide as a main component is used. Asanother example of a metal oxide target, an oxide semiconductor targetcontaining In, Ga, and Zn (a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1[mol %], that is, In:Ga:Zn=1:1:0.5 [atom %]) can be used. In addition,as the oxide semiconductor target containing In, Ga, and Zn, a targethaving a composition ratio of In:Ga:Zn=1:1:1 [atom %] or In:Ga:Zn=1:1:2[atom %] can be used. The filling rate of the oxide semiconductor targetis equal to or greater than 90% and equal to or less than to 100%,preferably equal to or greater than 95% and equal to or less than 99.9%.With the use of the oxide semiconductor target with a high filling rate,a dense oxide semiconductor film is formed. A target containing SiO₂ at2 wt % or more and 10 wt % or less may be used.

The oxide semiconductor film is deposited in a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or an atmosphere containing arare gas (typically argon) and oxygen.

It is preferable to use a high-purity gas from which an impurity such ashydrogen, water, hydroxyl, or hydride is removed to by a level of partsper million or parts per billion, as a sputtering gas.

The substrate 1 is held in a deposition chamber kept under reducedpressure, a sputtering gas from which an impurity such as hydrogen,water, hydroxyl, or hydride is removed is introduced while removingresidual moisture in the deposition chamber, and the above target isused, whereby the oxide semiconductor film is formed. In order to removeresidual moisture from the deposition chamber, an adsorption-type vacuumpump is preferably used. For example, a cryopump, an ion pump, or atitanium sublimation pump is preferably used. The evacuation unit may bea turbo pump provided with a cold trap. In a deposition chamber which isevacuated using a cryopump, for example, hydrogen atoms, compoundsincluding hydrogen atoms such as water (H₂O) (more preferably, compoundsincluding carbon atoms as well), or the like are exhausted; therefore,the concentration of impurities contained in the oxide semiconductorfilm which is deposited in the deposition chamber can be reduced. Thesubstrate 1 may be heated to a temperature of less than 400° C. when theoxide semiconductor film is formed.

An example of the deposition condition is as follows: the temperature ofthe substrate 1 is room temperature, the distance between the substrateand the target is 110 mm, the pressure is 0.4 Pa, the direct current(DC) power is 0.5 kW, and the atmosphere is an atmosphere containingoxygen and argon (the flow ratio of oxygen to argon is 15 sccm:30 sccm).Note that a pulse direct current (DC) power is preferable becauseparticles can be reduced and the film thickness can be uniform. Thepreferable thickness of the oxide semiconductor film is equal to orgreater than 2 nm and equal to or less than 200 nm. Note that anappropriate thickness differs depending on an oxide semiconductormaterial, and the thickness may be set as appropriate depending on thematerial.

Then, the oxide semiconductor film is etched with a resist mask that isformed by a photolithography process, so that the oxide semiconductorfilm 5 is formed (FIG. 11A). Note that the etching of the oxidesemiconductor film may be dry etching, wet etching, or both dry etchingand wet etching.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused.

As an etchant used for wet etching, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, an ammonia peroxidemixture (hydrogen peroxide water at 31 wt %: ammonia water at 28 wt %:water=5:2:2), or the like can be used. In addition, ITO07N (manufacturedby KANTO CHEMICAL CO INC.) may also be used.

Heat treatment (also referred to as a first heat treatment) may beperformed on the oxide semiconductor film 5. The temperature of the heattreatment is equal to or greater than 400° C. and equal to or less than750° C., preferably equal to or greater than 400° C. and less than thestrain point of the substrate 1. The heat treatment time may be 0.5hours to 5 hours. By this heat treatment, hydrogen atoms contained inthe oxide semiconductor film 5 can be removed. For example, in anelectric furnace, heat treatment is performed on the oxide semiconductorfilm 5 in a nitrogen atmosphere at 450° C. for one hour or at 650° C.for one hour, and then, the oxide semiconductor film is not exposed tothe air, whereby hydrogen atoms can be prevented from entering the oxidesemiconductor film 5. Note that the oxide semiconductor film 5 iscrystallized in some cases through the first heat treatment. TEManalysis in the case where heat treatment is performed at 650° C. forone hour is shown in Example 2.

The apparatus for the heat treatment is not limited to the electricfurnace and may be provided with a device for heating the oxidesemiconductor film 5, using heat conduction or heat radiation from aheating element such as a resistance heating element. For example, anRTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermalanneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus canbe used. An LRTA apparatus is an apparatus for heating the oxidesemiconductor film 5 by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high pressure sodium lamp, or a highpressure mercury lamp. A GRTA apparatus is an apparatus for heattreatment using a high-temperature gas. As the gas, an inert gas whichdoes not react with the oxide semiconductor film 5 by heat treatment,such as nitrogen or a rare gas such as argon is used.

GRTA heats the oxide semiconductor film 5 for 2 minutes to 5 minutes inan inert gas atmosphere at a temperature as high as 650° C. to 700° C.With GRTA, high-temperature heat treatment for a short period of timecan be achieved.

Note that it is preferable that in the heat treatment, hydrogen, water,hydroxyl, or hydride, or the like be not contained in nitrogen or a raregas such as helium, neon, or argon. It is preferable that the purity ofnitrogen or the rare gas such as helium, neon, or argon which isintroduced into a heat treatment apparatus be set to be 6N (99.9999%) orhigher, preferably 7N (99.99999%) or higher (that is, the impurityconcentration is 1 ppm or lower, preferably 0.1 ppm or lower).

As described above, depending on the conditions of the heat treatment orthe material of the oxide semiconductor film, the oxide semiconductorfilm may be crystallized to be a microcrystalline film or apolycrystalline film. For example, the oxide semiconductor film maycrystallize to become a microcrystalline oxide semiconductor film havinga degree of crystallization of 90% or more, or 80% or more. Further,depending on the conditions of the first heat treatment or the materialof the oxide semiconductor film, the oxide semiconductor film may becomean amorphous oxide semiconductor film which does not contain acrystalline component. Furthermore, the oxide semiconductor film maybecome an oxide semiconductor film in which a microcrystalline portion(with a grain diameter equal to or greater than 1 nm and equal to orless than or 20 nm (typically equal to or greater than 2 nm and equal toor less than 4 nm)) is mixed in an amorphous oxide semiconductor.

Moreover, after the above heat treatment is performed, a second heattreatment may be performed in atmosphere of oxygen which does notcontain hydrogen, water, hydroxyl, hydride, or the like, or in anatmosphere of nitrogen and oxygen which does not contain hydrogen,water, hydroxyl, hydride, or the like. Since oxygen in the oxidesemiconductor film 5 is removed in some cases through the above firstheat treatment, oxygen can be introduced in the oxide semiconductor film5 through the second heat treatment.

Note that it is preferable to perform reverse sputtering beforeformation of a conductive film in the following step so that a resistresidue and the like attached to surfaces of the oxide semiconductorfilm 5 and the first gate insulating film 4 can be removed.

The conductive film is formed by a sputtering method over the oxidesemiconductor film 5 and the first gate insulating film 4. The abovepretreatment is preferably performed before the film formation so thathydrogen, hydroxyl, and moisture are contained as little as possible inthe conductive film. The conductive film is etched with a resist maskthat is formed by a photolithography step, so that the electrode 6A (oneof a source electrode and a drain electrode) and the electrode 6B (theother of the source electrode and the drain electrode) are formed (FIG.11B).

Next, plasma treatment using a gas such as N₂O, N₂, or Ar is performedto remove water or the like adsorbed on a surface of the oxidesemiconductor film 5 which is exposed. Alternatively, plasma treatmentis performed using a mixture gas of oxygen and argon.

After the plasma treatment is performed, the second gate insulating film7 is formed over the oxide semiconductor film 5, the electrode 6A, andthe electrode 6B, without exposure to the air (FIG. 11C). In a regionwhere the oxide semiconductor film 5 is not in contact with any one ofthe electrode 6A and the electrode 68, the oxide semiconductor film 5 isin contact with the second gate insulating film 7. Further, the secondgate insulating film 7 covers the electrode 6A and the electrode 6B.

As the second gate insulating film 7, a silicon oxide film having adefect, for example, is formed as follows: a sputtering gas containinghigh-purity oxygen in which hydrogen and moisture are removed isintroduced while heating the substrate 1 at room temperature or at atemperature of less than 100° C., and a silicon semiconductor target isused.

For example, a silicon oxide film is formed by a pulsed DC sputteringmethod with use of a boron-doped silicon target which has a purity of 6N(a resistivity of 0.01 cm, in which the distance between the substrateand the target is 89 mm, the pressure is 0.4 Pa, the direct current (DC)power is 6 kW, and the atmosphere is an oxygen atmosphere (theproportion of oxygen flow is 100%). Note that instead of a silicontarget, quartz (preferably, synthetic quartz) can be used as a targetused when the silicon oxide film is formed. As a sputtering gas, oxygenor a mixed gas of oxygen and argon is used.

In this case, it is preferable that the second gate insulating film 7 beformed while residual moisture in the treatment chamber is removed. Thisis for preventing hydrogen, hydroxyl, or moisture from being containedin the oxide semiconductor film 5 and the second gate insulating film 7.In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used.

Heat treatment may be performed at 100° C. to 400° C. while the secondgate insulating film 7 and the oxide semiconductor film 5 are in contactwith each other. The second gate insulating film 7 has a lot of defects.By this heat treatment, an impurity such as hydrogen, moisture,hydroxyl, or hydride included in the oxide semiconductor film 5 can bediffused into the second gate insulating film 7 and the number ofhydrogen atoms contained in the oxide semiconductor film 5 can befurther reduced.

A conductive film is formed over the second gate insulating film 7. Theconductive film is etched with a resist mask that is formed by aphotolithography step, so that the second gate electrode 8 is formed(FIG. 11C). Note that the conductive film is preferably formed by asputtering method in order to prevent hydrogen, hydroxyl, or moisturefrom being contained in the conductive film. In the case where thesecond gate electrode 8 is electrically connected to the first gateelectrode 3, a contact hole is formed in the second gate insulating film7 and the first gate insulating film 4 before the conductive film isformed. The contact hole reaches the first gate electrode 3. Theconductive film is formed after the contact hole is formed. The secondgate electrode 8 is formed of the conductive film.

An organic resin film is formed over the second gate insulating film 7as a planarizing film. The organic resin film is formed by aspin-coating method or the like. The organic resin film is selectivelyetched to form the organic resin film 9 (FIG. 12A).

An opening reaching the electrode 6B is formed in the organic resin film9 and the second gate insulating film 7. The pixel electrode 10 isformed in the opening (FIG. 12B). The pixel electrode 10 is electricallyconnected to the electrode 6B.

A display medium is provided over the pixel electrode 10, the secondgate electrode 8, and the second gate insulating film 7 (FIGS. 14A, 15A,16A, and 22A).

An insulating film 21 serving as a passivation film may be formed tohave a thickness of 10 nm to 200 nm over the second gate electrode 8 andthe second gate insulating film 7 (FIG. 13). A silicon oxide film, asilicon nitride film, or the like can be used for the insulating film21. As the insulating film 21, a silicon oxide film, a silicon nitridefilm, or the like, is formed as follows: a sputtering gas containinghigh-purity oxygen in which hydrogen and moisture are removed isintroduced while heating the substrate 1 at room temperature or at atemperature of less than 100° C., and a silicon semiconductor target isused.

The organic resin film 9 is formed over the insulating film 21 as aplanarizing film. The opening reaching the electrode 6B is formed in theorganic resin film 9, the insulating film 21, and the second gateinsulating film 7. The pixel electrode 10 is formed in the opening (FIG.13). A display medium is provided over the pixel electrode 10 and theinsulating film 21 (FIGS. 14B, 15B, 16B, and 22B).

Hereinafter, as a display medium, liquid crystal, EL, and electronicpaper are described. Note that the display medium is not limited toliquid crystal, EL, and electronic paper.

<Liquid Crystal>

FIG. 14A illustrates a case where the display medium is liquid crystal.A first alignment film 22 is formed over the pixel electrode 10. Thefirst alignment film 22 is also formed over and in contact with thesecond gate electrode 8 and the second gate insulating film 7. A liquidcrystal layer 23 is formed over the first alignment film 22. A secondalignment film 24 is formed over the liquid crystal layer 23. The liquidcrystal layer 23 is provided between the first alignment film 22 and thesecond alignment film 24. An electrode 25 serving as a counter electrodeis formed over the second alignment film 24 and a substrate 26 servingas a counter substrate is formed over the electrode 25. The firstalignment film 22 and the second alignment film 24 are not necessarilyformed as long as the liquid crystal layer 23 is aligned. A spacer maybe provided in order to keep a cell gap.

As for a mode which is used in the liquid crystal display, a twistednematic (TN) mode, an in-plane-switching (IPS) mode, a fringe fieldswitching (FFS) mode, a multi-domain vertical alignment (MVA) mode, apatterned vertical alignment (PVA) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, or the like can be employed. Alternatively, a bluephase mode may be used.

The first alignment film 22, the second alignment film 24, the electrode25, and the substrate 26 are formed using known materials.

The first alignment film 22, the liquid crystal layer 23, the secondalignment film 24, and the electrode 25 are formed by known methods.

As described above, the insulating film 21 serving as a passivation filmmay be formed (FIG. 14B). The first alignment film 22 is formed over andin contact with the insulating film 21 and the pixel electrode 10.

<EL>

FIG. 15A illustrates a case where the display medium is EL. An EL layer31 is formed over the pixel electrode 10. An electrode 32 serving as acounter electrode is formed over the EL layer 31. The EL layer 31 andthe electrode 32 need not to be formed over the second gate electrode 8and the second gate insulating film 7. A sealing material 33 is formedover and in contact with the second gate electrode 8, the second gateinsulating film 7, and the electrode 32. A substrate 34 serving as asealing substrate is formed over the sealing material 33.

EL layers are classified into an organic EL layer and an inorganic ELlayer. Further, inorganic EL layers are classified into a dispersiontype inorganic EL and a thin-film type inorganic EL.

In the case of an organic EL layer, the EL layer 31 includes, forexample, a hole-injection layer, a hole-transport layer, alight-emitting layer, an electron-transport layer, and anelectron-injection layer. In the light-emitting layer, a dopant materialis added to a host material. As for a dopant material, a phosphorescentlight-emitting material or a fluorescent light-emitting material isused.

In the case of a dispersion type inorganic EL, the EL layer 31 includesa light-emitting layer in which particles of the light-emitting materialare dispersed in a binder. In the case of a thin-film type inorganic EL,the EL layer 31 has a structure in which a light-emitting layer isprovided between dielectric layers.

As for the sealing material 33, an ultraviolet curable resin or athermosetting resin can be used, as well as an inert gas such asnitrogen or argon. For example, PVC (polyvinyl chloride), acrylic,polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl butyral), orEVA (ethylene vinyl acetate) can be used.

The EL layer 31, the electrode 32, the sealing material 33, and thesubstrate 34 are formed using known materials. In addition, the EL layer31, the electrode 32, and the sealing material 33 are formed by knownmethods.

As described above, the insulating film 21 serving as a passivation filmmay be formed (FIG. 15B). The sealing material 33 is formed over and incontact with the insulating film 21 and the electrode 32.

<Electronic Paper>

FIG. 16A illustrates a case where the display medium is electronicpaper. In FIG. 16A, a twisting ball display system is used. The twistball display system refers to a method in which spherical particles eachcolored in black and white which are arranged between the pixelelectrode 10 and the electrode 44 are used for a display element, and apotential difference is generated between the pixel electrode 10 and theelectrode 44 to control the orientation of the spherical particles, sothat display is performed.

A filler 43 such as a resin is formed over and in contact with the pixelelectrode 10, the second gate electrode 8, and the second gateinsulating film 7. In the filler 43, each of spherical particlesincluding a cavity 42 is provided. The cavity 42 includes a black region41 and a white region 40. The electrode 44 serving as a counterelectrode is formed over the filler 43. A substrate 45 serving as acounter substrate is provided over the electrode 44.

Instead of the twist ball display system, an electrophoretic displaysystem can be used (FIG. 22A). An electronic ink layer 51 is formed overand in contact with the pixel electrode 10, the second gate electrode 8,and the second gate insulating film 7. In the electronic ink layer 51, amicrocapsule 52 having a diameter of about 10 μm to 200 μm in whichpositively charged white microparticles and negatively charged blackmicroparticles are encapsulated, is provided. An electrode 53 serving asa counter electrode is formed over the electronic ink layer 51 and asubstrate 54 serving as a counter substrate is provided over theelectrode 53.

The filler 43, the cavity 42, the electrode 44, the substrate 45, theelectronic ink layer 51, the microcapsule 52, the electrode 53 and thesubstrate 54 are formed using known materials. In addition, the filler43, the cavity 42, the electrode 44, the electronic ink layer 51, themicrocapsule 52, and the electrode 53 are formed by known methods.

As described above, the insulating film 21 serving as a passivation filmmay be formed (FIG. 16B and FIG. 22B). The filler 43 or the electronicink layer 51 is formed over and in contact with the insulating film 21and the pixel electrode 10.

A display device according to the present invention can be applied to avariety of electronic devices (including an amusement machine). Examplesof electronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a digital camera, a digital video camera, a digital photo frame, amobile phone (also referred to as a portable telephone or a mobile phonedevice), a portable game console, a portable information terminal, anaudio reproducing device, a large-sized game machine such as a pachinkomachine, and the like.

FIG. 23A illustrates an example of a mobile phone. A mobile phone 1100is provided with a display portion 1102 incorporated in a housing 1101,operation buttons 1103, an external connection port 1104, a speaker1105, a microphone 1106, and the like. The above display device isprovided for the display portion 1102.

FIG. 23B illustrates an example of a portable information terminal. Theportable information terminal includes a housing 2800 and a housing2801. The housing 2800 includes a display panel 2802, a speaker 2803, amicrophone 2804, a pointing device 2806, a camera lens 2807, an externalconnection terminal 2808, and the like. The housing 2801 includes akeyboard 2810, an external memory slot 2811, and the like. Further, anantenna is incorporated in the housing 2801. The above display device isprovided for the display panel 2802.

FIG. 24A illustrates an example of a television set. In the televisionset 9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610. The above display device is provided for the displayportion 9603 and the display portion 9607.

FIG. 24B illustrates an example of a digital photo frame. For example,in the digital photo frame 9700, a display portion 9703 is incorporatedin a housing 9701. The display portion 9703 can display a variety ofimages. For example, the display portion 9703 can display data of animage taken with a digital camera or the like and function as a normalphoto frame. The above display device is provided for the displayportion 9703.

FIG. 25 illustrates a portable game console including a housing 9881 anda housing 9891 which are jointed with a connector 9893 so as to beopened and closed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively. Theabove display device is provided for the display portion 9883.

FIG. 26 illustrates an example of an electronic book. For example, theelectronic book 2700 includes a housing 2701 and a housing 2703. Thehousing 2701 and the housing 2703 are combined with a hinge 2711 so thatthe electronic book 2700 can be opened and closed with the hinge 2711 asan axis. With such a structure, the electronic book 2700 can operatelike a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 26) and graphics can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 26).

FIG. 26 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, or the like can also beprovided on the same face as the display portion of the housing.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, a terminal that can be connected to various cables such asan AC adapter and a USB cable, or the like), a recording mediuminsertion portion, and the like may be provided on the back surface orthe side surface of the housing. Moreover, the electronic book 2700 mayhave a function of an electronic dictionary.

The above display device is provided for the display portion 2705 andthe display portion 2707.

EXAMPLE 1

In this example, a method for calculating the carrier density of anoxide semiconductor film is described with reference to FIGS. 17, 18Aand 18B.

First, the structure of a sample used in a C-V (Capacitance-Voltage)measurement is described with reference to FIG. 17.

A titanium film 503 with a thickness of 300 nm was formed by asputtering method over a glass substrate 501 and a titanium nitride film505 with a thickness of 100 nm was formed by a sputtering methodthereover.

As an oxide semiconductor film 507 in which the number of hydrogen atomswere reduced, an In—Ga—Zn—O film with a thickness of 2000 nm was formedby a sputtering method over the titanium nitride film 505. Thedeposition condition at this time was as follows: as the sputtering gas,Ar at a flow rate of 30 sccm and oxygen at a flow rate of 15 seem wereused, the distance between the target and the substrate was 60 mm, thedirect current (DC) power was 0.5 kW, and the deposition atmospheretemperature was room temperature.

Next, a silicon oxynitride film 509 with a thickness of 300 nm wasformed by a CVD method and a silver film 511 with a thickness of 300 nmwas formed thereover.

Next, C-V measurement results of the sample at 300 K are shown in FIG.18A. According to the measurement results shown in FIG. 18A, a curve ofC⁻² relative to the voltage is shown in FIG. 18B. Here, the carrierdensity can be obtained by substituting the gradient of a curve of C⁻²in a weak inversion state of the sample into Formula 2. In addition, inFIG. 188, a curve of C is indicated by a solid line and the gradient ofC⁻² in a weak inversion state is indicated by a broken line. Thegradient of the curve was −1.96×10¹⁸ F⁻²V⁻¹.

$\begin{matrix}{N_{d} = {{- \left( \frac{2}{{\mathbb{e}ɛ}_{0}ɛ} \right)}/\frac{\mathbb{d}\left( {1/C} \right)^{2}}{\mathbb{d}V}}} & \left\lbrack {{FORMULA}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Note that e is the amount of charge per electron, ∈ is relativepermittivity, ∈₀ is permittivity of vacuum, and N_(d) is carrierdensity.

According to Formula 2, the carrier density of the oxide semiconductorin this example was 6×10¹⁰ cm⁻³. Accordingly, it is found that thecarrier density of the oxide semiconductor in this example is extremelylow.

EXAMPLE 2

In this example, with regard to an oxide semiconductor film in which thenumber of hydrogen atoms are reduced by heat treatment, the results ofTEM analysis are described.

First, a method for manufacturing a sample is described.

An oxide semiconductor film was formed by a sputtering method over asubstrate 601.

Here, as the substrate 601, EAGLE XG substrate (manufactured by CorningIncorporated) was used. As the oxide semiconductor film, an In—Ga—Zn—Ofilm 603 was deposited with the use of an oxide semiconductor target ofIn₂O₃:Ga₂O₃:ZnO=1:1:1. The sample is referred to as sample B which is acomparative example. The number of hydrogen atoms is not reduced in thesample B.

Next, heat treatment was performed on the In—Ga—Zn—O film 603 at 650° C.for one hour in a nitrogen gas atmosphere with the use of an electricfurnace, whereby the number of hydrogen atoms was reduced. TheIn—Ga—Zn—O film on which heat treatment was performed is referred to asan oxide semiconductor film 605. The sample is referred to as sample A.

A cross section of each sample was observed with a high-resolutiontransmission electron microscope (TEM: “H9000-NAR” manufactured byHitachi, Ltd.) at an acceleration voltage of 300 kV to examine acrystalline state. FIGS. 19A and 19B show cross-sectional photographs ofthe sample A and FIGS. 20A and 20B show cross-sectional photographs ofthe sample B. Note that FIG. 19A and FIG. 20A are low magnificationphotographs (two million-fold magnification) and FIG. 19B and FIG. 20Bare high magnification photographs (four million-fold magnification).

A continuous lattice image was observed in a superficial portion in across section of the sample A on which heat treatment was performed inan electric furnace at 650° C. for one hour, as shown in FIGS. 19A and19B. In particular, in the high magnification photograph of FIG. 19B, aclear lattice image is observed in a region surrounded by a white frame,and the existence of crystals whose crystal axes are aligned isindicated. Accordingly, it is found that the superficial portion of theIn—Ga—Zn—O-based non-single-crystal film is crystallized by performingheat treatment in an electric furnace at 650° C. for one hour and acrystalline region is provided. Note that a clear continuous latticeimage was not observed in a region except for the superficial portionand a state where microcrystalline particles exist here and there in anamorphous region was found. The particle size of the microcrystals wasequal to or greater than 2 nm and equal to or less than or 4 nm.

On the other hand, a clear lattice image was not observed in any regionin a thickness direction in the cross-sectional photographs of FIGS. 20Aand 20B (the sample B) and it was found that the sample B is amorphous.

Next, FIG. 21A shows an enlarged photograph of the superficial portionof the sample A on which heat treatment was performed in an electricfurnace at 650° C. for one hour and FIG. 21B shows an electrondiffraction pattern of a crystalline region. Directional arrows 1 to 5indicating directions where lattice images are aligned are illustratedin the enlarged photograph of the superficial portion (FIG. 21A), andneedle crystals are grown in a direction perpendicular to a surface ofthe film. The electron diffraction patterns shown in FIGS. 21B, 21C,21D, 21E, and 21F were observed at positions indicated by the arrows 1,2, 3, 4, and 5, respectively, and a c-axis orientation is found.

From the results of analysis, it is found that the superficial portionof the sample on which heat treatment is performed in an electricfurnace at 650° C. for one hour has a crystal region.

This application is based on Japanese Patent Application serial no.2009-276454 filed with Japan Patent Office on Dec. 4, 2009, the entirecontents of which are hereby incorporated by reference.

1. A display device comprising: a first gate electrode; a first gateinsulating film over the first gate electrode; an oxide semiconductorfilm over the first gate insulating film; a source electrode and a drainelectrode over the oxide semiconductor film, wherein the sourceelectrode and the drain electrode are electrically connected to theoxide semiconductor film; a second gate insulating film over the sourceelectrode, the drain electrode and the oxide semiconductor film; asecond gate electrode over the second gate insulating film; an organicresin film having flatness over the second gate insulating film; a pixelelectrode over the organic resin film having flatness, wherein the pixelelectrode is electrically connected to either the source electrode orthe drain electrode; a first alignment film over and in contact with thesecond gate electrode and the pixel electrode; a liquid crystal layerover the first alignment film; a second alignment film over the liquidcrystal layer; a counter electrode over the second alignment film; and acounter substrate over the counter electrode, wherein a concentration ofhydrogen atoms contained in the oxide semiconductor film and measured bysecondary ion mass spectrometry is less than 1×10¹⁶ cm⁻³.
 2. A displaydevice comprising: a first gate electrode; a first gate insulating filmover the first gate electrode; an oxide semiconductor film over thefirst gate insulating film; a source electrode and a drain electrodeover the oxide semiconductor film, wherein the source electrode and thedrain electrode are electrically connected to the oxide semiconductorfilm; a second gate insulating film over the source electrode, the drainelectrode and the oxide semiconductor film; a second gate electrode overthe second gate insulating film; an organic resin film having flatnessover the second gate insulating film; a pixel electrode over the organicresin film having flatness, wherein the pixel electrode is electricallyconnected to either the source electrode or the drain electrode; an ELlayer over the pixel electrode; a counter electrode over the EL layer; asealing material over and in contact with the second gate electrode andthe counter electrode; and a counter substrate over the sealingmaterial, wherein a concentration of hydrogen atoms contained in theoxide semiconductor film and measured by secondary ion mass spectrometryis less than 1×10¹⁶ cm⁻³.
 3. A display device comprising: a first gateelectrode; a first gate insulating film over the first gate electrode;an oxide semiconductor film over the first gate insulating film; asource electrode and a drain electrode over the oxide semiconductorfilm, wherein the source electrode and the drain electrode areelectrically connected to the oxide semiconductor film; a second gateinsulating film over the source electrode, the drain electrode and theoxide semiconductor film; a second gate electrode over the second gateinsulating film; an organic resin film having flatness over the secondgate insulating film; a pixel electrode over the organic resin filmhaving flatness, wherein the pixel electrode is electrically connectedto either the source electrode or the drain electrode; and a filler overand in contact with the second gate electrode and the pixel electrode,wherein a spherical particle including a cavity is provided in thefiller, the cavity contains a black region and a white region, and aspace around the cavity is filled with a liquid, wherein a concentrationof hydrogen atoms contained in the oxide semiconductor film and measuredby secondary ion mass spectrometry is less than 1×10¹⁶ cm⁻³.
 4. Adisplay device comprising: a first gate electrode; a first gateinsulating film over the first gate electrode; an oxide semiconductorfilm over the first gate insulating film; a source electrode and a drainelectrode over the oxide semiconductor film, wherein the sourceelectrode and the drain electrode are electrically connected to theoxide semiconductor film; a second gate insulating film over the sourceelectrode, the drain electrode and the oxide semiconductor film; asecond gate electrode over the second gate insulating film; an organicresin film having flatness over the second gate insulating film; a pixelelectrode over the organic resin film having flatness, wherein the pixelelectrode is electrically connected to either the source electrode orthe drain electrode; and an electronic ink layer over and in contactwith the second gate electrode and the pixel electrode, wherein theelectronic ink layer comprises a microcapsule in which positivelycharged white microparticles and negatively charged black microparticlesare encapsulated, wherein a concentration of hydrogen atoms contained inthe oxide semiconductor film and measured by secondary ion massspectrometry is less than 1×10¹⁶ cm⁻³.
 5. A display device according toclaim 1, wherein the organic resin film does not overlap the second gateelectrode.
 6. A display device according to claim 2, wherein the organicresin film does not overlap the second gate electrode.
 7. A displaydevice according to claim 3, wherein the organic resin film does notoverlap the second gate electrode.
 8. A display device according toclaim 4, wherein the organic resin film does not overlap the second gateelectrode.
 9. A display device according to claim 1, further comprisinga passivation film over the second gate electrode and the second gateinsulating film.
 10. A display device according to claim 2, furthercomprising a passivation film over the second gate electrode and thesecond gate insulating film.
 11. A display device according to claim 3,further comprising a passivation film over the second gate electrode andthe second gate insulating film.
 12. A display device according to claim4, further comprising a passivation film over the second gate electrodeand the second gate insulating film.
 13. A display device according toclaim 1, wherein a carrier density of the oxide semiconductor film isless than 1×10¹² cm⁻³ at 300 K.
 14. A display device according to claim2, wherein a carrier density of the oxide semiconductor film is lessthan 1×10¹² cm⁻³ at 300 K.
 15. A display device according to claim 3,wherein a carrier density of the oxide semiconductor film is less than1×10¹² cm⁻³ at 300 K.
 16. A display device according to claim 4, whereina carrier density of the oxide semiconductor film is less than 1×10¹²cm⁻³ at 300 K.
 17. A display device according to claim 1, wherein theoxide semiconductor film comprises a material selected from the groupconsisting of In—Sn—Ga—Zn—O, In—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O,Sn—Ga—Zn—O, Al—Ga—Zn—O, Sn—Al—Zn—O, In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O,Sn—Mg—O, In—Mg—O; In—O, Sn—O, and Zn—O.
 18. A display device accordingto claim 2, wherein the oxide semiconductor film comprises a materialselected from the group consisting of In—Sn—Ga—Zn—O, In—Ga—Zn—O,In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O, Sn—Al—Zn—O, In—Zn—O,Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, In—Mg—O; In—O, Sn—O, and Zn—O.
 19. Adisplay device according to claim 3, wherein the oxide semiconductorfilm comprises a material selected from the group consisting ofIn—Sn—Ga—Zn—O, In—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O,Al—Ga—Zn—O, Sn—Al—Zn—O, In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O,In—Mg—O; In—O, Sn—O, and Zn—O.
 20. A display device according to claim4, wherein the oxide semiconductor film comprises a material selectedfrom the group consisting of In—Sn—Ga—Zn—O, In—Ga—Zn—O, In—Sn—Zn—O,In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O, Sn—Al—Zn—O, In—Zn—O, Sn—Zn—O,Al—Zn—O, Zn—Mg—O, Sn—Mg—O, In—Mg—O; In—O, Sn—O, and Zn—O.